Method for Removal of Reactive Sulfur from Insulating Oil

ABSTRACT

A method for removing sulfur from insulating oil. The oil is exposed to at least one sulfur scavenging material and at least one polar sorbent.

FIELD OF THE INVENTION

The present invention relates to methods and devices for treatment ofoil containing reactive sulfur-containing compounds.

BACKGROUND OF THE INVENTION

Insulating oils are used in power transformers, distributiontransformers and reactors. These oils often contain traces of reactivesulfur compounds, especially thiols (a.k.a. mercaptans). The reactivesulfur compounds may react with copper, forming copper mercaptides. Thecopper mercaptides can decompose further, leading to the formation ofcopper(I)sulfide, Cu₂S. The net reactions could be as shown below:

Cu₂O+2RSH=>2 CuSR+H₂O

2CuSR=>Cu₂S+RSR

where —SH is a thiol group (or mercaptan), —R is an alkyl group.

Other sulfurorganics, especially sulfides, can also be active, either bydirect reaction with copper or via conversion to thiols.

Cu₂S is insoluble in oil and may form deposits, especially on surfacesof cellulose material. Once deposition of Cu₂S has started the processis believed to be self-catalytic. The surface selectively adsorbs thereactants and intermediates, and also catalyzes the decomposition ofintermediates to Cu₂S.

SUMMARY OF THE INVENTION

A method for removing sulfur-containing compounds from insulating oil isprovided. The method includes exposing the oil to at least one sulfurscavenging material and exposing the oil to at least one polar sorbent.

A system for removing sulfur-containing compounds from insulating oil isalso provided. The system includes a pump operative to pump the oilthrough the system, at least one sulfur scavenger containing device, andat least one sulfur sorbent containing device.

Further objectives and advantages, as well as the structure and functionof exemplary embodiments will become apparent from a consideration ofthe description, drawings, and examples.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages of the invention will beapparent from the following, more particular description of an exemplaryembodiment of the invention, as illustrated in the accompanying drawingswherein like reference numbers generally indicate identical,functionally similar, and/or structurally similar elements.

FIG. 1 illustrates an embodiment of a winding with spacers separatingturns;

FIG. 2 represents a graph that illustrates a typical voltage transientgenerated by line or commutation fault;

FIG. 3 a represents a schematic diagram illustrating one embodiment of asystem according to the present invention;

FIG. 3 b represents a schematic diagram illustrating another embodimentof a system according to the present invention;

FIG. 4 represents a schematic diagram illustrating a further embodimentof a system according to the present invention; and

FIG. 5 represents a schematic diagram illustrating a another embodimentof a system according to the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments of the invention are discussed in detail below. Indescribing embodiments, specific terminology is employed for the sake ofclarity. However, the invention is not intended to be limited to thespecific terminology so selected. While specific exemplary embodimentsare discussed, it should be understood that this is done forillustration purposes only. A person skilled in the relevant art willrecognize that other components and configurations can be used withoutparting from the spirit and scope of the invention.

Power transformers, distribution transformers and reactors usuallyinclude an insulation system consisting of oil and cellulose. These twocomponents have been used for a long time due to their relatively lowprice and good performance. The dielectric strength of such aninsulation system is strongly dependent on its insulating properties.

During operation of such devices, copper(I)sulfide may be deposited onthe cellulose of the insulation. This can lower the initiation level forpartial discharges (PD). With deposition in areas of the windings, withhigh electrical stresses and under certain operating conditions,especially the abundance of transients, PD activity may lead todegradation of the solid insulation and ultimately to dielectricbreakdown.

An example of an area where the electrical stress is high is betweenturns in the windings. This turn-to-turn insulation is typically builtup by conductor insulation, which may include paper wrapping, forexample, and sometimes also spacers separating the conductors from eachother. FIG. 1 illustrates one example of a winder with spacersseparating the turns. The conductors are insulated with paper wrapping.Both the conductor insulation and the spacers will then be verysensitive for copper(I)sulfide deposits.

From time to time, transformers are subjected to voltage transientscoming from lightning, so-called lightning impulse. Other sources ofvoltage transients are, for example, commutation faults or line faultsin high voltage direct current (HVDC) systems. FIG. 2 is a graph thatillustrates a relationship between voltage and time, showing a typicalvoltage transient generated by line or commutation fault. These faultsare especially dangerous due to that the repetitive nature of thesefaults. In some installations, hundreds of commutation faults and linefaults can occur within a typical 24 hour period.

Reactions leading to Cu₂S formation can be prevented or suppressed byremoving or reducing active copper and sulfur containing components.However, conventional insulating oil processing techniques, such asreconditioning and reclaiming have little or no effect. Reclaiming,which is typically carried out by treating the oil with a sorbent forpolar contaminants, such as Fullers earth or alumina, has as its primarypurpose to remove oxidation products from aged oil, and restore it to acondition similar to that of new oil. Copper mercaptides and othercopper-organic compounds can be removed with this process. However, theeffect on mercaptan content is limited, and that on content of sulfidesand disulfides even less.

The present invention provides both methods for removal of reactivesulfur from oil as well as apparatus for carrying out removal processes.These potentially harmful reactive sulfur compounds, which can includesulfur organics can be removed and/or converted to more easily removedcompounds by treating the oil with material with a high reactivitytowards sulfur. Such materials with a high reactivity toward sulfur maybe referred to as sulfur scavengers. Reaction with sulfur scavengers canconvert the sulfur containing compounds into more easily removedcompounds. For example, the reactive sulfur containing compounds may beconverted to insoluble metal sulfides, or oil-soluble metal mercaptides.These mercaptides could then be captured by polar sorbent materials,e.g. of the types used in normal oil reclaiming. Accordingly, the oilmay also or alternatively be treated with at least one polar sorbentmaterial.

The oil may be exposed to the sulfur scavenger(s) and/or polarsolvent(s) in any order. Also, the oil may be exposed to the sulfurscavenger(s) and/or polar solvent(s) more than one time. The exposuremay take place in any order. For example, the oil may be exposed to thescavenger(s) twice and then to the sorbent(s). A combination of one ormore sulfur scavenger materials and one or more polar sorbent materialscan be used. The best results may be obtained if the last exposure ofthe oil to scavenger(s) is followed by exposure to polar sorbent(s).This can remove any oil soluble products produced by the exposure to thescavenger. However, good results may be obtained with any of thetreatment schemes herein.

Exposure to scavengers and sorbents may be carried out in a number ofdifferent ways. For example, the scavengers and/or sorbents may besuspended in the oil. According to another example, the scavengersand/or sorbents may be contained in a vessel, such as a column. The oilmay be exposed to the scavengers and/or sorbents in other ways as well.

One way to realize this combined treatement is to first let the oil flowthough a device with a large area of such materials that act as sulfurscavengers. For example, the oil may be directed through a column filledwith one or more sulfur scavengers described below. Then, the oil mayflow though a vessel filled with a polar sorbent material. Unsolublereaction products, such as copper sulfide, may be retained on thescavenger material. On the other hand, soluble reaction products, suchas copper mercaptides, may be transported with the oil to the sorbentmaterial, where they are adsorbed.

If the exposure in done by suspending the sulfur scavengers and sorbentmaterials in oil the exposure time is from a few minutes to severalhours. For example, the exposure time may be about 1 minute to about 10hours. Typically, the exposure time is about 5 minutes to about 60minutes. This time may include exposure to all sorbents and scavengers.Alternatively, the oil may be exposed to scavenger(s) for such a periodof time and then sorbent(s) for such a period of time. In fact, if theoil is exposed to scavengers and/or sorbents a number of times, eachexposure period may be for a length of time as described above. Theexact time period may depend upon the level of reactivesulfur-containing compounds in the untreated oil and/or the reduction inthe level of such compounds that is desired to achieve.

If the exposure is carried out in a flow system with the reagentscontained in columns typical flow rates would be from about 500 to about5000 liters/hours, with typical contact times from a fraction of aminute to several minutes. For example, the contact time may be about 30seconds to about 10 mintues.

In order to more efficiently remove sulfur-containing compounds that thesorbent(s) are less efficient at removing, the oil may be exposed to oneor more scavengers. The exposure to scavenger(s) may take place prior toexposing the oil to the sorbent(s). Also, one or more scavengers may becombined with one or more sorbents in one vessel. In fact, the eachscavenger and/or sorbent could be in a separate container. One or morescavengers may be in one container and one or more sorbents may be inone container. The arrangement of the scavengers and sorbents may dependupon their nature. For example, since elemental copper and copper oxidehave different selectivity their combined effect is sometimes greaterthan that of only one of them. In cases where the sorbent material isreactivated instead of discarded after use, at least some of the sulfurscavenger materials may have to be kept in separate containers, sincethey may be affected by the reactivation. Such a situation isillustrated with the embodiment shown in FIG. 3 b, which is discussedbelow in greater detail. To permit the scavenger(s) to more efficientlyremove sulfur-containing compounds, the scavenger(s) may be provided ina manner that provides a large surface area including the scavenger(s).

The large surface area may be accomplished in a variety of differentways. For example, the scavenger(s) may be provided on a substratehaving a high surface area. Additionally or alternatively, thescavenger(s) may be provided in a form that has a high surface area.

One or more scavengers may be utilized according to the presentinvention. The scavengers may include any material that can remove orconvert the desired sulfur-containing compounds from the oil into morepolar compounds. Examples of sulfur scavengers can include elementalcopper and other metals that easily form sulfides, such as ferrousmetals, for example Fe, Ni, and Cr. The metals can also be in anoxidized state, such as a salt. Metal oxides may be particularly useful.One metal salt that may be utilized is copper(I)oxide Cu₂O. Thoseskilled in the art may be aware of other scavengers that may beutilized. The above list is meant to be exemplary and not exhaustive.

The form of the scavenger(s) may vary, depending upon the composition ofthe scavenger. For example, metals and metal salts can be in the shapeof pieces of wire, wool, cuttings, granulate, sponge, or any otherpreparation with a large surface area. Other scavengers may havedifferent forms and/or may be provided on a substrate.

The oil may be exposed to the sulfur scavengers in any suitablestructure. For example, the scavenger(s) may be contained within atreatment column. The column may be filled with one or more scavengersin forms described above or one or more substrates in and/or on whichone or more scavengers may be arranged.

Different scavengers and/or sorbent materials may be utilized to removedifferent sulfur-containing compounds. For example, soluble reactionproducts can be removed or reduced by treating the oil with one or moresorbents for polar contaminants. Examples of soluble sulfur-containingcompounds includes thiols and copper mercaptides. Examples of sorbentsfor polar contaminants can include phosphates, silicates and/or metaloxides. Examples of silicates can include Fullers earth and zeolites.Examples of metal oxides can include copper(I)oxide (Cu₂O), titaniumoxide (TiO₂)or alumina (Al₂O₃). Examples of phosphates can includehydroxy apatite. Those skilled in the art may be aware of other sorbentsthat may be utilized. The above list is meant to be exemplary and notexhaustive.

The sorbent(s) may be used and reactivated. In fact, the sorbent may beused and reactivated several times. In this case such large amounts ofcopper and other metals may accumulate on the sorbent that it may alsoact a sulfur scavenger.

Although sorbents for polar contaminants may be highly efficient forremoving dissolved copper and some thiols, other components such assulfides, disulfides, less polar mercaptans and insoluble reactionproducts may be less efficiently removed by a sorbent treatment.Examples of compounds that are less efficiently removed by sorbenttreatment can include sulfides, disulfides, and some thiols.

Treatment of the oil may be preferably carried out at a temperaturehigher than normal bulk oil temperature. In other words, the oil may betreated at a temperature higher than the normal operating temperature ofthe oil. For example, the treatment may be carried out at a temperatureof about 60° C. or greater. The temperature range may also be about 70°C. or greater and even about 80° C. or greater. In fact, temperatures ofupto about 100° C. or more may be utilized. The treatment may also besuccessful at lower temperatures and even at ambient temperature.

The efficiency of sulfur scavengers, as well as the sorption process,are typically greater at higher temperature. However, at too hightemperatures decomposition of the oil may take place and/or otherunwanted side effects may occur. The optimal temperature for carryingout the process typically depends upon the condition and composition ofthe oil. The oxygen content of the oil may affect the temperatures thatmay be utilized. For example, low oxygen content may permit highertemperatures to be utilized in the sulfur removal process while notdestroying the oil.

Exposure to temperatures above the normal operating temperature of theoils maybe carried out at any point during the treatment. Typically, thetemperature of the oil is raised prior to exposure to the sorbent(s)and/or the scavenger(s).

Treatment methods may also include filtering the oil. Filtration mayoccur prior to exposing the oil to scavenger(s) and/or sorbent(s). Alongthese lines, the oil may be filtered prior to any treatment withscavenger(s) and/or sorbent(s). Alternatively and/or additionally, theoil may be filtered after all treatment with scavenger(s) and/orsorbent(s). Furthermore, the oil may also or alternatively be filteredafter any treatment of the oil with scavenger(s) and/or sorbent(s).

Treatment methods may be implemented in a variety of ways. For example,a commercially available mobile oil reclaiming plant may be utilized. Insuch a case, a top layer of sorbent in one or more columns typicallyincluded in such plants may be replaced with one or more sulfurscavenging materials. Each column may be altered in this manner.

The treatment may be carried out in a continuous mode, tank-to-tank, oron-line on equipment. However, it is also possible to carry outtreatment methods by contacting the scavenger and sorbent materials inbulk, in two consecutive steps, with filtration after each step.

The present invention also includes a system for treating oil. A systemincludes at least one structure including at least one sulfur scavenger.The oil is contacted with the sulfur scavenger in the structure. Thesystem also includes at least one structure including at least one polarsorbent. The oil is contacted with the sorbent in the structure. Thestructures can include a treatment column or other structure. In someembodiments, a single structure may contain both scavenger(s) andsorbent(s). The single structure may include one or more separatingstructures for separating the scavenger(s) and sorbent(s) from eachother and/or from other scavenger(s) and sorbent(s) in the structure. Asystem may include multiple structures that contain scavenger(s) and/orsorbent(s). The structures may be arranged in parallel and/or in series.

The system may include a pump operative to pump the oil through thesystem. A heater may be included to heat the oil to temperatures abovethe typical operating temperature of the oil. The heater may be arrangedat any location within the system to provide the heated oil wheredesired. One or more filters may be arranged with in the system tofilter the oil as desired. For example, filters could be arranged priorto and/or subsequent to any treatment with scavenger(s) and/orsorbent(s). One or more filters could also be arranged prior to or afterindividual treatment with scavenger(s) and/or sorbent(s). Along theselines, the oil could be filtered after treatment with scavenger(s) andbefore treatment with sorbent(s). The oil may be transferred to aconservator or expansion vessel after treatment and before beingreturned to a transformer or other tank. The oil may also be returned bya valve in a transformer tank or oil cooling system. The system may beconnected directly to a transformer tank. Alternatively, the systemcould receive the oil to be treated after the oil is removed from atransformer tank.

FIG. 3 a illustrates an embodiment of a system for on-line treatment ofoil according to the present invention. The system is arranged to treatoil in a transformer. The system 10 is connected to a transformer tank12. A pump 14 pumps oil from the tank to be treated. The oil is pumpedfrom the tank to a heater 16 for heating the oil. The oil then moves toa single column 18 containing a first sulfur scavenger 1 and a secondsulfur scavenger 2. A sorbent 3 is also arranged in the column. A filter20 filters the oil after treatment by the scavengers and the sorbent.After filtration, the oil passes to a conservator 22. The oil is thenreturned to the tank. The tank, pump, heater, column, filter andconservator are connected by pipes. In this embodiment, the first sulfurscavenger 1 can be copper shavings, the second sulfur scavenger 2 can becopper(I)oxide wire, and the sorbent can be activated Fullers earth oralumina.

FIG. 3 b illustrates another embodiment of a system according to thepresent invention. The system shown in FIG. 3 b also for on-linetreatment of oil in a transformer, using a combination of two sulfurscavenger materials and one sorbent material. However, this embodimentmay be utilized in applications where one scavenger material would beaffected by the reactivation process used for the sorbent material. Oneexample of such an application would be when sulfur scavenger 1 iselemental copper, sulfur scavenger 2 is a mix of copper oxides, and thesorbent is reactivated by in-situ incineration. The reactivation in thisembodiment takes place in reactivation zone 24.

FIG. 4 illustrates an embodiment that may be utilized for tank-to-tankprocessing. Along these lines, the embodiment includes a first tank 26containing oil to be treated. The oil is transferred from the first tank26 to a column 28 containing a mixture of one or more scavengers and oneor more sorbents. After exposure to the scavenger(s) and sorbent(s) inthe column, the oil is transferred to a second tank 30, which holds thetreated oil.

FIG. 5 illustrates an embodiment of the present invention that includesa column train that may be utilized when oil to be treated contains ametal passivator that could prevent reaction with the scavengermaterials and that can be removed by a sorbent as a first step. Alongthese lines, the embodiment shown in FIG. 5 includes a first column 32containing Fullers earth. The second column 34 includes copper shavings.The third column 36 contains copper oxide and the fourth column 38Fullers earth. The oil flows from column to column as indicated by thearrows. For such a case a mixed bed could also be used.

Typically, methods according to the present invention are carried our onoil filled in transformers, without de-energizing them. A continuousprocess, such as may be carried out with the embodiment shown in FIG. 3,may be the best option for carry out the treatment. In this case, theoil may be pumped and heated, keeping scavengers and sorbents incolumns, and using a sorbent that can be in-situ reactivated. Such acontinuous treatment may be carried out with a modified commerciallyavailable mobile reclaiming plants, such as are

1. A method for removing sulfur from insulating oil, the methodcomprising: exposing the oil to at least one sulfur scavenging material;and exposing the oil to at least one polar sorbent.
 2. The methodaccording to claim 1, further comprising: filtering the oil.
 3. Themethod according to claim 1, further comprising: heating the oil priorto exposing the oil to the sulfur scavenging material and exposing theoil to the polar sorbent.
 4. The method according to claim 1, whereinthe at least one sulfur scavenging material is provided on a highsurface area substrate.
 5. The method according to claim 4, wherein theat least one sulfur scavenging material is provided in a column.
 6. Themethod according to claim 1, wherein the at least one sulfur scavengingmaterial comprises at least one of elemental copper or other sulfideforming metal or metal salts.
 7. The method according to claim 6,wherein metal salt comprises copper oxide.
 8. The method according claim6, wherein the at least one sulfur scavenging material is provided in ashape include at least one of wire, wool, cuttings, granules, foil andsponge.
 9. The method according to claim 1, wherein the at least onesulfur scavenging material retains insoluble reaction products.
 10. Themethod according to claim 1, wherein the at least one polar sorbentabsorbs soluble reaction products.
 11. The method according to claim 10,wherein the soluble reaction products comprise copper mercaptides. 12.The method according to claim 1, wherein the at least one polar sorbentcomprises at least one of Fullers earth and alumina.
 13. The methodaccording to claim 1, further comprising: reactivating the at least onepolar sorbent.
 14. The method according to claim 13, wherein the atleast one polar sorbent is reactivated a plurality of times.
 15. Themethod according to claim 1, wherein the method is carried out at atemperature higher than an operating temperature of the oil.
 16. Themethod according to claim 1, wherein the method is carried out at atemperature of at least 70° C.
 17. The method according to claim 1,wherein the method is carried out in a continuous mode.
 18. The methodaccording to claim 1, wherein the method is carried out on-line.
 19. Themethod according to claim 1, wherein the oil is exposed to the at leastone sulfur scavenging material a plurality of times, the method furthercomprising: filtering the oil after each time that the oil is exposed tothe at least one sulfur scavenging material.
 20. A system for removingsulfur from insulating oil, the system comprising: a pump operative topump the oil through the system; at least one sulfur scavengercontaining structure; and at least one polar sorbent containingstructure.
 21. The system according to claim 20, further comprising: atleast one filter operative to filter the oil after treatment with the atleast one sulfur scavenger containing structure or the at least onesulfur sorbent containing structure.
 22. The system according to claim20, further comprising: a heater operative to heat the oil prior totreatment with the at least one sulfur scavenger containing structure orthe at least one sulfur sorbent containing structure.
 23. The systemaccording to claim 20, wherein the pump is interconnected with atransformer tank.
 24. The system according to claim 20, comprising aplurality of sulfur scavenger containing structures.
 25. The systemaccording to claim 20, wherein the oil passes through the at least onesulfur scavenger containing structure prior to passing through the atleast one sulfur sorbent containing structure.
 26. The system accordingto claim 20, wherein the at least one sulfur scavenger containingstructure and the at least one sulfur sorbent containing structure areparts of a single container.